Two-step mixing process for producing an absorbent polymer

ABSTRACT

A process for producing an absorbent polymer including a first mixing event, in which a plurality of absorbent polymer particles ( 1 ) are mixed with a liquid ( 2 ) and a second mixing event, in which the liquid ( 2 ) is homogenized within the interior of the polymer particles. The polymer particles ( 1 ) in the first mixing event are mixed with a speed such that the kinetic energy of the individual polymer particles ( 1 ) is on average larger than the adhesion energy of the individual polymer particles ( 1 ), and the polymer particles ( 1 ) in the second mixing event are stirred at a lower speed than in the first mixing event. The different speeds effect a fluidization of the polymer particles ( 1 ), which prevents a clumping of the polymer particles ( 1 ) during the mixing event. The absorbent polymers thus produced are distinguished by a particularly rapid swelling behavior.

This application is a national stage application under 35 U.S.C. 371 ofinternational application no. PCT/EP2003/011830 filed Oct. 24, 2003,which is based on German Application No. DE 102 49 822.9, filed Oct. 25,2002, and claims priority thereto.

BACKGROUND OF THE INVENTION

The invention is related to a process for producing an absorbentpolymer, an absorbent polymer obtainable by this process, an absorbentpolymer, a composite, a process for producing the composite, a compositeobtainable by this process, chemical products as well as the use of theabsorbent polymer and of the composite.

In order to form so-called “superabsorbent” polymers a polymerization ofdifferent types of normally water-soluble monomers, often however alsoof water insoluble co-monomers, together with the presence ofcross-linkers, is necessary. The addition of the cross-linkers occursduring or after the polymerization. Superabsorbent polymers of this typeare lightly cross-linked, water insoluble hydrogel polymers, which havea large capacity for water absorption in the dry and in the essentiallywater-free state. The absorbed quantities of water can constitute amultiple of the weight of the superabsorbent polymer itself.

Because of this high absorption capacity superabsorbent polymers aresuitable for water absorbent structures and objects, such as for examplebaby diapers, incontinence products or sanitary napkins.

Although the ratio of the absorbed weight of water to the dry weight ofthe polymer, i.e. the absorption capacity, is sufficient forapplications of this type, the rate with which the water is absorbed,i.e. the rate of absorption, is limited and for many application casesunsatisfactory. In many cases the high absorption capacity of thesuperabsorbent polymers has no effect, since the slow absorption rateprevents a sufficiently fast absorption of the water and thereby rendersdifficult the use of the polymers in hygiene applications.

WO-A-98/49221 describes a process for improving the properties ofabsorbent polymer particles, in which the polymer particles which, forexample during a secondary cross-linking reaction, have been heatedbeforehand for at least 10 minutes at least 170° C., are mixed with anaqueous additive solution. Upon mixing the polymers with an aqueoussolution, however, agglomeration of the polymer particles occurs, whichaccording to the teaching of WO-A-98/49221 can only be prevented byaddition of additives to the aqueous solution, for example in the formof singly or multiply charged metal cations.

In general the object of the present invention is to overcome thedisadvantages arising from the state of the art.

It is in particular an object of the present invention to specify aprocess by which the superabsorbent polymers can be produced, whichpolymers have an increased rate of absorption towards water withoutappreciably reducing their absorption capacity. In addition this processshould be carried out in a simple way and be possible without the use ofadditives, in particular without the use of organic additives.

A further object of the invention consists especially in providing asuperabsorbent polymer, composites which comprise superabsorbentpolymers of this type and chemical products which comprisesuperabsorbent polymers or composites of these types, wherein thesuperabsorbent polymer has an increased rate of absorption towardswater. In addition the absorption properties of these polymers undermechanical load, which occur in particular upon transport of the polymerparticles in conveyor systems, should not be negatively influenced. Inparticular the absorption capacity of the polymer under a force load dueto mechanical load should not be decreased at all if possible or atleast be only slightly reduced (=mechanical stability).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is better understood by a reading of the DetailedDescription of the Invention along with a review of the drawings inwhich:

FIG. 1 illustrates both steps of the process according to the inventionfor producing an absorbent polymer.

FIG. 2 illustrates a collision between a polymer particle and twostuck-together polymer particles during the first mixing event.

FIG. 3 illustrates the test set-up for determination of the absorptiontime (Acquisition Time) in cross-section.

FIG. 4 illustrates a detailed view of the test set-up for determinationof the absorption time (Acquisition Time).

FIG. 5 illustrates a depiction of the dimensions of individual elementsof the test set-up for determination of the absorption time (AcquisitionTime).

FIGS. 6 and 7 illustrate experimental set-up for determination of therewet value.

DETAILED DESCRIPTION OF THE INVENTION

These objects are achieved by the objects of the category-formingclaims. Advantageous embodiments and further forms which can occurindividually or in combination are the object of the respectiveindependent claims.

In the process according to the invention for producing an absorbentpolymer, comprising a first mixing event, in which a plurality ofabsorbent polymer particles are mixed with a liquid, and a second mixingevent, in which the liquid is distributed, preferably homogenized,within the polymer particles, the polymer particles are mixed in thefirst mixing event at a speed such that the kinetic energy of theindividual polymer particles is on average greater than the adhesionenergy between the individual polymer particles, and the polymerparticles in the second mixing event are stirred at a lower speed thanin the first mixing event.

In the first mixing event the polymer particles are wetted at theirsurface with the liquid. The polymer particles are stirred in the firstmixing event at a higher speed than in the second mixing event.

The high speed effects a fluidization of the polymer particles, whichprevents the polymer particles from remaining adhered to each other andclumping. The high speed leads, in the case of an adhesion of twopolymer particles to each other, to dissociation of both clumped polymerparticles from each other through a collision with a third polymerparticle or through collision of the clumped polymer particles with awall e.g. of the mixer.

The condition that the kinetic energy of the individual polymerparticles is on average larger than the adhesion energy between theindividual polymer particles results in the polymer particles beingfound predominantly as individuals and not as composites, agglomerationsor drops. Since the adhesion energy is smaller than the kinetic energy,the polymer particles are found in constant movement as individuals.

The liquid is equally distributed via the movement of the polymerparticles and the numerous impacts with each other. The surfaces of thepolymer particles are evenly wetted with liquid.

In the first mixing event large dynamic forces arise which are caused bythe high speeds of the polymer particles. The quasi-static forces arehowever small in comparison.

By means of the second mixing event, the polymer particles mature in themanner that the liquid distributes itself within the polymer particles.

A clumping of the polymer particles or an agglomeration of the polymerparticles is prevented by the low speed of the polymer particles in thesecond mixing event. Preferably the selected speed in the second mixingevent is so small that the quasi-static compression force is kept low.As a result a clumping is impeded.

In one embodiment of the process according to the invention there is amaturation event between the first mixing event and the second mixingevent, in which the polymer particles are moved against each other lessthan in the first and second mixing events. Preferably the polymerparticles rest in the maturation event. The polymer particles preferablyremain in the maturation event for a period of time from about 1 secondto about 10 hours, preferably about 1.1 seconds to about 60 minutes andparticularly preferably about 1.5 seconds to about 20 minutes.

In another embodiment of the process according to the invention theaverage speed of the polymer particles in the first mixing event amountsto between about 8 and about 80 m/sec, in particular about 10 and about70 m/sec, above all preferably between about 15 and about 60 m/sec.

With such high average speeds in the first mixing event sufficientlylarge kinetic energies are reached for the typical densities of theabsorbent polymer granulates or the weights of the individual polymerparticles. These large kinetic energies act in opposition to the effortof the polymer particles to remain adhered together. The effort of thepolymer particles to remain adhered together is characterized by theadhesion energy. The size of the adhesion energy depends upon aplurality of parameters, among others on the cohesion of the liquid, theadhesion between liquid and polymer particles as well as the cohesionbetween two (optionally partially swollen) polymer particles. Theadhesion energy is essentially given by how much energy is necessary toseparate from each other two adhered together polymer particles.

The second mixing event occurs with sufficiently small quasi-staticforces. Large static forces result in a clumping of the polymerparticles. In order to prevent high static forces appropriate mixertypes such as e.g. screw mixers are used. Furthermore the second mixingevent can occur in a drum which according to one embodiment has mixingpaddles on the drum wall. Such mixer types are inexpensive andeconomical in use.

The level of the speeds in the first mixing event as well as the speedsin the second mixing event depend on the particular type of theabsorbent polymer. The speeds can be determined by statisticalconsiderations as well as by simple measurements. For example theminimum speed necessary for the first mixing process is determined fromthe average kinetic energy necessary to overcome the adhesion energy.The adhesion energy is the energy necessary to separate two clumpedtogether polymer particles from each other. It can be determined by ameasurement. Using these speeds for the respective mixing events has theeffect that the polymer particles do not remain adhered to each other.If the polymer particles do not clump, the surface of the polymergranulates remains maximal, whereby a rapid absorption of water iseffected.

In a further embodiment of the process according to the invention theaverage speed of the polymer particles in the second mixing eventamounts to less than about 3 m/sec, particularly under about 0.3 m/sec,preferably under about 0.03 m/sec. These low speeds prevent anagglomeration of the polymer particles.

In a further embodiment of the process according to the invention forproducing an absorbent polymer the Froude number in the first mixingprocess amounts to between about 1 and about 50, in particular betweenabout 1.1 and about 45, further preferred between about 1.5 and about40, particularly preferred between about 1.7 and about 33 and even morepreferred between about 10 and about 33. In a particular embodiment ofthe process according to the invention the Froude number in the firstmixing event amounts to at least about 5, preferred at least about 10,particularly preferred at least about 15, more preferred at least about20 and above all even more preferred at least about 25, wherebypreferably a value of about 60 is not exceeded. If too high Froudenumbers or speeds are selected the polymer particles aredisadvantageously altered.

The Froude number in the second mixing event amounts according to theinvention to preferably between about 0.001 and about 1, in particularbetween about 0.01 and about 0.2, preferably between about 0.08 andabout 0.03.

The Froude number is a characteristic characterizing number, whichdetermines for rotation mixers the mixing effect of the mixerindependently from the dimensions of the mixer. The Froude numberFr=(R×ω²)/g gives the centrifugal acceleration of the products to bemixed, normalized with respect to acceleration due to gravity, wherein Ris the radius of the rotation mixer, ω the rotation frequency of therotation mixer and g the gravitational constant (=9.81 m/s²). Byspecifying the Froude number an apparatus-independent mixing effect canbe specified independent of the individual set-up of the mixer byspecifying a normalized acceleration.

The Froude number necessary for the fluidization in the first mixingevent for typical superabsorbent polymers is, according to experience,reached between about 1.7 and about 33. For polymer particles, whichhave a lower density and thereby a lower mass, assuming that theadhesion energy is the same, correspondingly larger Froude numbers areto be applied. The lighter the polymer particles are, the higher theirspeeds must be, so that an adhesion or a clumping is broken up throughthe movement of the polymer particles. For heavier polymer particlescorrespondingly lower speeds or smaller Froude numbers are necessary.

The second mixing event demands lower speeds, which correspond tosmaller Froude numbers. What is important in the second mixing event, isthat only low static forces develop, which result in a clumping of thepolymer particles.

In a particular embodiment of the process according to the invention thepolymer particles are back-mixed in the first mixing event. Back-mixingmeans that older polymer particles, which have already been mixed for awhile, are put together with polymer particles which are freshly wetwith liquid. This happens preferably in a continual mixing process. Insuch a mixing process the flow of the new polymer particles entering themixer is superposed by a flow of polymer particles already present inthe mixer, counter to the entering flow. Preferably the ratio of thecounter flow to the flow of the newly entering polymer particles, i.e.back-mixing, amounts to about 5% to about 50%, preferably about 10% toabout 30% and particularly preferably about 15% to about 25%. Theback-mixing is effected by a suitable configuration of the mixing organ(propeller) of the mixer. The back-mixing causes a particularly evenwetting of the polymer particles with liquid to be achieved. Aparticularly even homogenization of the liquid within the polymerparticles contributes to the polymer particles being characterized byfast liquid absorption.

In a special embodiment of the process the mean residence time of thefirst mixing event amounts to between about 5 and about 200 sec, inparticular between about 10 and about 100 sec, preferably between about20 and about 60 sec. By the mean residence time of the first mixingevent is understood preferably the length of the time interval in whichthe polymer particles on average during the first mixing event are mixedwith a liquid with a speed such that the kinetic energy of the polymerparticles is on average larger than the adhesion energy between theindividual polymer particles. A particularly economical operation of themixer used for the first mixing event is achieved through the short meanresidence time of the polymer particles in the first mixing event. Inaddition the formation of fines through mechanical abrasion is preventedby this short duration.

In a particular embodiment of the process according to the invention thestatic pressure build-up during the first mixing event amounts to lessthan about 0.1 bar, in particular less than about 0.05 bar, preferablyless than about 0.01 bar. By means of these low static pressures aclumping of the polymer particles is prevented. By preventing a clumpinga large surface area is provided which allows a rapid liquid absorption.

In order to increase the absorption rate, preferably water or an aqueoussolution is added as the liquid. In a special embodiment of theinvention the liquid comprises additives, in particular alcohols. Inanother special embodiment the liquid comprises no additives. In thisspecial embodiment of the process according to the invention the liquidis in particular free from additives such as singly or multiply chargedmetal cations or water soluble organic substances with a viscositywithin a range between about 200 and about 300 centistokes, wherein theliquid is in particular free from those substances which are concretelyspecified as additives in WO-A-98/49221. Free from additives in thesense of the above inventions means that the liquid comprises, besidesthe liquid as main component, these additives in quantities of less thanabout 500 ppm, preferably less than about 100 ppm, particularlypreferably less than about 10 ppm, above all even more preferably lessthan about 1 ppm and most preferably less than about 0.1 ppm.

The polymer particles used in the process according to the inventionpreferably have an average particle size according to ERT 420.1-99within a range from 10 to 10000 μm, particularly preferably within arange from about 50 to about 5000 μm and above all preferably within arange from about 100 to about 1000 μm.

It is further preferred that the polymer particles used in the processaccording to the invention have:

-   (α1) about 0.1 to about 99.999 wt. %, preferably about 20 to about    98.99 wt. % and particularly preferably about 30 to about 98.95 wt.    % of polymerized, ethylenically unsaturated, acidic group-containing    monomers or salts thereof, or polymerized, ethylenically unsaturated    monomers containing a protonated or a quaternary nitrogen, or    mixtures thereof, wherein at least ethylenically unsaturated, acidic    group-containing monomers, preferably acrylic acid comprising    mixtures are particularly preferred,-   (α2) 0 to about 70 wt. %, preferably about 1 to about 60 wt. % and    particularly preferably about 1 to about 40 wt. % of polymerized,    ethylenically unsaturated monomers which can be co-polymerized with    (α1),-   (α3) about 0.001 to about 10 wt. %, preferably about 0.01 to about 7    wt. % and particularly preferably about 0.05 to about 5 wt. % of one    or more cross-linkers,-   (α4) 0 to about 30 wt. %, preferably about 1 to about 20 wt. % and    particularly preferably about 5 to about 10 wt. % of water soluble    polymers, as well as-   (α5) 0 to about 20 wt. %, preferably about 0.01 to about 7 wt. % and    particularly preferably about 0.05 to about 5 wt. % of one or more    additives, wherein the sum of the component weights (α1) to (α5)    amounts to 100 wt. %.

The monoethylenically unsaturated, acidic group-containing monomers (α1)can be partially or fully, preferably partially neutralized. Themonoethylenically unsaturated acidic groups are neutralized preferablyto at least about 25 mol %, particularly preferred to at least about 50mol % and even more preferred to about 50 to about 90 mol %. Theneutralization of the monomers (α1) can occur before and also after thepolymerization. Furthermore, the neutralization can be carried out withalkali metal hydroxides, alkaline earth metal hydroxides, ammonia, aswell as carbonates and bicarbonates. In addition any further base whichforms a water soluble salt with the acid is conceivable. A mixedneutralization with different bases is also conceivable. Neutralizationwith ammonia or with alkali metal hydroxides is preferred, with sodiumhydroxide or with ammonia is particularly preferred.

Furthermore the free acidic groups can predominate in a polymer, so thatthis polymer has a pH value lying in the acidic region. This acidicwater-absorbing polymer can be at least partially neutralized by apolymer with free basic groups, preferably amino groups, which polymeris basic in comparison to the acidic polymer. These polymers aredescribed in the literature as “Mixed-Bed Ion-Exchange AbsorbentPolymers” (MBIEA-polymers) and are disclosed in WO 99/34843 amongothers. As a rule MBIEA-polymers produce a compound which comprises onthe one hand basic polymers, which are in a position to exchange anionsand on the other hand a polymer which is acidic in comparison to thebasic polymer, said acidic polymer being in a position to exchangecations. The basic polymer has basic groups and is typically obtained bypolymerization of monomers which carry basic groups or groups which canbe converted into basic groups. With these monomers those which haveprimary, secondary or tertiary amines or the corresponding phosphines orat least two of the above functional groups are concerned above all. Tothis group of monomers belong particularly ethylenamine, allylamine,diallylamine, 4-aminobutene, alkyloxycycline, vinylformamide,5-aminopentene, carbodiimide, formaldacine, melamine and the like, aswell as secondary or tertiary amine derivatives thereof.

Preferred monoethylenically unsaturated, acidic group-containingmonomers (α1) are acrylic acid, methacrylic acid, ethacrylic acid,α-chloroacrylic acid, α-cyanoacrylic acid, β-methylacrylic acid(crotonic acid), α-phenylacrylic acid, β-acryloxypropionic acid,sorbinic acid, α-chlorosorbinic acid, 2′-methylisocrotonic acid,cinnamic acid, p-chlorocinnamic acid, 0-stearic acid, itaconic acid,citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleicacid, fumaric acid, tricarboxythylene and maleic acid anhydride, whereacrylic acid and methacrylic acid and above all acrylic acid areparticularly preferred.

Besides these carboxylate group-containing monomers, further preferredmonoethylenically unsaturated acidic group-containing monomers (α1) areethylenically unsaturated sulfonic acid monomers or ethylenicallyunsaturated phosphonic acid monomers.

Preferred ethylenically unsaturated sulfonic acid monomers areallylsulfonic acid or aliphatic or aromatic vinylsulfonic acids oracrylic or methacrylic sulfonic acids. Preferred aliphatic or aromaticvinylsulfonic acids are vinylsulfonic acid, 4-vinylbenzylsulfonic acid,vinyltoluenesulfonic acid and styrenesulfonic acid. Preferred acrylic ormethylacrylic sulfonic acids are sulfoethyl(meth)acrylate,sulfopropyl(meth)acrylate and 2-hydroxy-3-methacryloxypropylsulfonicacid. As (meth)acrylamidoalkylsulfonic acid,2-acrylamido-2-methylpropansulfonic acid is preferred.

Additionally preferred are ethylenically unsaturated phosphonic acidmonomers, such as vinylphosphonic acid, allylphosphonic acid,vinylbenzylphosphonic acid, (meth)acrylamidoalkylphosphonic acids,acrylamidoalkyldiphosphonic acids, phosphonomethylated vinylamines and(meth)acrylphosphonic acid derivatives.

Preferred ethylenically unsaturated monomers (α1) containing aprotonated nitrogen are dialkylaminoethyl(meth)acrylate-hydrochloridesin the protonated form, for exampledimethylaminoethyl(meth)acrylate-hydrochloride ordimethylaminoethyl(meth)acrylate-hydrosulfate, as well asdialkylaminoalkyl(meth)acrylamides in the protonated form, for exampledimethylaminoethyl(meth)acrylamide-hydrochloride,dimethylaminopropyl(meth)acrylamide-hydrochloride,dimethylaminopropyl(meth)acrylamide-hydrosulfate ordimethylaminoethyl(meth)acrylamide-hydrosulfate.

Preferred ethylenically unsaturated monomers (α1) containing aquatemated nitrogen are dialkylammoniumalkyl(meth)acrylates inquaternated form, for exampletrimethylammoniumethyl(meth)acrylate-methosulfate ordimethylethylammoniumethyl(meth)acrylate-ethosulfate as well as(meth)acrylamidoalkyldialkylamines in quaternated form, for example(meth)acrylamidopropyltrimethylammonium chloride,trimethylammoniumethyl(meth)acrylate chloride and(meth)acrylamidopropyltrimethylammonium sulfate.

According to the invention it is preferred that the polymer comprise atleast about 50 wt. %, preferably at least about 70 wt. % and above allpreferably at least 90 wt. % carboxylate group-containing monomers.According to the invention it is particularly preferred that the polymercomprise at least about 50 wt. %, preferably at least about 70 wt. %acrylic acid, which is neutralized preferably to at least about 20 mol %and particularly preferably to at least about 50 mol %.

Preferred monoethylenically unsaturated monomers (α2) which can beco-polymerized with (α1) are acrylamides and (meth)acrylamides.

Possible (meth)acrylamides besides acrylamide and methacrylamide arealkyl-substituted (meth)acrylamides or aminoalkyl substitutedderivatives of (meth)acrylamides such as N-methylol(meth)acrylamide,N,N-dimethylamino(meth)acrylamide, dimethyl(meth)acrylamide ordiethyl(meth)acrylamide. Possible vinylamides are for exampleN-vinylamides, N-vinylformamides, N-vinylacetamides,N-vinyl-N-methylacetamide, N-vinyl-N-methylformamides, vinylpyrrolidone.Among these monomers acrylamide is particularly preferred.

Further preferred monoethylenically unsaturated monomers (α2) which arecopolymerizable with (α1) are water dispersible monomers. Preferredwater dispersible monomers are acrylic acid esters and methacrylic acidesters, such as methyl(meth)acrylate, ethyl(meth)acrylate,propyl(meth)acrylate or butyl(meth)acrylate, as well as vinylacetate,styrene and isobutylene.

Preferred cross-linkers (α3) according to the invention are compoundswhich have at least two ethylenically unsaturated groups in one molecule(cross-linker class I), compounds which have at least two functionalgroups which can react with functional groups of the monomers (α1) or(α2) in a condensation reaction (=condensation cross-linkers), anaddition reaction or a ring-opening reaction (cross-linker class II),compounds which have at least one ethylenically unsaturated group and atleast one functional group which can react with functional groups of themonomers (α1) or (α2) in a condensation reaction, an addition reactionor a ring-opening reaction (cross-linker class III), or polyvalent metalcations (cross-linker class (IV). Thereby a cross-linking of the polymeris achieved with the compounds of cross-linker class I by radicalpolymerization of the ethylenically unsaturated groups of thecross-linker molecules with the monoethylenically unsaturated monomers(α1) or (α2), while with the compounds of cross-linker class II and thepolyvalent metal cations of cross-linker class IV a cross-linking of thepolymer is achieved via condensation reaction of the functional groups(cross-linker class II) or via electrostatic interaction of thepolyvalent metal cation (cross-linker class IV) with the functionalgroups of the monomer (α1) or (α2). With compounds of cross-linker classIII a cross-linking of the polymers is achieved correspondingly byradical polymerization of the ethylenically unsaturated groups or justas well by condensation reaction between the functional groups of thecross-linkers and the functional groups of the monomers (α1) or (α2).

Preferred compounds of cross-linker class I are poly(meth)acrylic acidesters, which have been obtained for example by conversion of a polyol,such as for example ethylene glycol, propylene glycol,trimethylolpropane, 1,6-hexanediol, glycerin, pentaerythritol,polyethyleneglycol or polypropyleneglycol, of an aminoalcohol, apolyalkylenepolyamine, such as for example diethylenetriamine ortriethylenetetraamine, or of an alkoxidized polyol with acrylic acid ormethacrylic acid. Further preferred compounds of cross-linker class Iare polyvinyl compounds, poly(meth)allyl compounds, (meth)acrylic acidesters of a monovinyl compound or (meth)acrylic acid esters of amono(meth)allyl compound, preferably of the mono(meth)allyl compounds ofa polyol or of an aminoalcohol. In this context reference is made to DE195 43 366 and DE 195 43 368.

As examples of compounds of cross-linker class I are namedalkenyldi(meth)acrylates, for example ethyleneglycoldi(meth)acrylate,1,3-propyleneglycoldi(meth)acrylate, 1,4-butyleneglycoldi(meth)acrylate,1,3-butyleneglycoldi(meth)acrylate, 1,6-hexanedioldi(meth)acrylate,1,10-decanedioldi(meth)acrylate, 1,12-dodecanedioldi(meth)acrylate,1,18-octadecanedioldi(meth)acrylate, cyclopentanedioldi(meth)acrylate,neopentylglycoldi(meth)acrylate, methylenedi(meth)acrylate orpentaerythritoldi(meth)acrylate, alkenyldi(meth)acrylamides, for exampleN-methyldi(meth)acrylamide, N,N′-3-methylbutylidenebis(meth)acrylamide,N,N′-(1,2-dihydroxyethylene)bis(meth)acrylamide,N,N′-hexamethylenebis(meth)acrylamide orN,N′-methylenebis(meth)acrylamide, polyalkoxydi(meth)acrylates, forexample diethyleneglycoldi(meth)acrylate,triethyleneglycoldi(meth)acrylate, tetraethyleneglycoldi(meth)acrylate,dipropyleneglycoldi(meth)acrylate, tripropyleneglycoldi(meth)acrylate ortetrapropyleneglycoldi(meth)acrylate, bisphenol-A-di(meth)acrylate,ethoxylated bisphenol-A-di(meth)acrylate, benzylidenedi(meth)acrylate,1,3-di(meth)acryloyloxypropanol-2, hydroquinonedi(meth)acrylate,di(meth)acrylate esters of trimethylolpropane, which are preferablyalkoxylated with 1 to 30 mol alkylene oxide per hydroxyl group,preferably ethoxylated, thioethyleneglycoldi(meth)acrylate,thiopropyleneglycoldi(meth)acrylate,thiopolyethyleneglycoldi(meth)acrylate,thiopolypropyleneglycoldi(meth)acrylate, divinyl ethers, for example1,4-butanedioldivinylether, divinyl esters, for example divinyladipate,alkanedienes, for example butadiene or 1,6-hexadiene, divinylbenzene,di(meth)allyl compounds, for example di(meth)allylphthalate ordi(meth)allylsuccinate, homo- and co-polymers ofdi(meth)allyldimethylammonium chloride and homo- and co-polymers ofdiethyl(meth)allylaminomethyl(meth)acrylateammonium chloride,vinyl(meth)acrylic compounds, for example vinyl(meth)acrylate,(meth)allyl(meth)acrylic compounds, for example(meth)allyl(meth)acrylate, (meth)allyl(meth)acrylate ethoxylated with 1to 30 mol ethylene oxide per hydroxyl group, di(meth)allylesters ofpolycarbonic acids, for example di(meth)allylmaleate,di(meth)allylfumarate, di(meth)allylsuccinate ordi(meth)allylterephthalate, compounds with 3 or more ethylenicallyunsaturated, radically polymerizable groups such as for exampleglycerine tri(meth)acrylate, (meth)acrylate esters of glycerinsethoxylated with preferably 1 to 30 mol ethylene oxide per hydroxylgroup, trimethylolpropanetri(meth)acrylate, tri(meth)acrylate esters oftrimethylolpropane which is alkoxylated preferably with 1 to 30 molalkylene oxide per hydroxide group, preferably ethoxylated,trimethacrylamide, (meth)allylidenedi(meth)acrylate,3-allyloxy-1,2-propanedioldi(meth)acrylate, tri(meth)allylcyanurate,tri(meth)allylisocyanurate, pentaerythritoltetra(meth)acrylate,pentaerythritoltri(meth)acrylate, (meth)acrylic acid esters ofpentaerythritol which is ethoxylated with preferably 1 to 30 molethylene oxide per hydroxyl group,tris(2-hydroxyethyl)isocyanuratetri(meth)acrylate, trivinyltrimellitate,tri(meth)allylamine, di(meth)allylalkylamines, for exampledi(meth)allylmethylamine, tri(meth)allylphosphate,tetra(meth)allylethylenediamine, poly(meth)allyl ester,tetra(meth)allyloxyethane or tetra(meth)allylammonium halides.

Preferred compounds of cross-linker class II are compounds which have atleast two functional groups which can react with the functional groupsof the monomers (α1) or (α2), preferably with acidic groups of themonomers (α1), in a condensation reaction (=condensation cross-linkers),in an addition reaction or in a ring opening reaction. Examples of thesefunctional groups of the compounds of cross-linker class II arepreferably alcoholic, amino, aldehyde, glycidic, isocyanate, carbonateor epichloro functions.

As examples of compounds of cross-linker class II are mentioned polyols,for example ethyleneglycol, polyethyleneglycols such asdiethyleneglycol, triethyleneglycol and tetraethyleneglycol,propyleneglycol, polypropyleneglycols such as dipropyleneglycol,tripropyleneglycol or tetrapropyleneglycol, 1,3-butanediol,1,4-butanediol, 1,5-pentanediol, 2,4-pentanediol, 1,6-hexanediol,2,5-hexanediol, glycerine, polyglycerin, trimethylolpropane,polyoxypropylene, oxyethylene-oxypropylene-block copolymer,sorbitan-fatty acid esters, polyoxyethylenesorbitan-fatty acid esters,pentaerythritol, polyvinylalcohol and sorbitol, aminoalcohols, forexample ethanolamine, diethanolamine, triethanolamine or propanolamine,polyamine compounds, for example ethylenediamine, diethylenetriamine,triethylenetetraamine, tetraethylenepentaamine orpentaethylenehexaamine, polyglycidyl ether compounds such asethyleneglycoldiglycidyl ether, polyethyleneglycoldiglycidyl ether,glycerinediglycidyl ether, glycerinepolyglycidyl ether,pentaerithritolpolyglycidyl ether, propyleneglycoldiglycidyl ether,polypropyleneglycoldiglycidyl ether, neopentylglycoldiglycidyl ether,hexanediolglycidyl ether, trimethylolpropanepolyglycidyl ether,sorbitolpolyglycidyl ether, phthalic acid diglycidyl ester, adipinicacid diglycidyl ether, 1,4-phenylenebis(2-oxazoline), glycidol,polyisocyanates, preferably diisocyanates such as2,4-toluenediioscyanate and hexamethylenediisocyanate, polyaziridinecompounds such as2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate],1,6-hexamethylenediethyleneurea anddiphenylmethane-bis-4,4′-N,N′-diethyleneurea, halogen epoxides forexample epichloro- and epibromohydrin and α-methylepichlorohydrin,alkylenecarbonates such as 1,3-dioxolane-2-one (ethylene carbonate),4-methyl-1,3-dioxolane-2-one (propylene carbonate),4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one,4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one,1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one,4,6-dimethyl-1,3-dioxane-2-one, 1,3-dioxolane-2-one,poly-1,3-dioxolane-2-one, polyquaternary amines such as condensationproducts from dimethylamines and epichlorohydrin. Further preferredcompounds of the cross-linker class II are in addition polyoxazolinessuch as 1,2-ethylenebisoxazoline, cross-linkers with silane groups suchas γ-glycidooxypropyltrimethoxysilane and γ-aminopropyltrimethoxysilane,oxazolidinones such as 2-oxazolidinone, bis- and poly-2-oxazolidinoneand diglycolsilicates.

Preferred compounds of class III are hydroxyl or amino group-containingesters of (meth)acrylic acid, such as for example2-hydroxyethyl(meth)acrylate, as well as hydroxyl or aminogroup-containing (meth)acrylamides, or mono(meth)allylic compounds ofdiols.

The polyvalent metal cations of the cross-linker class IV are derivedpreferably from singly or multiply charged cations. Particularlypreferred doubly charged cations are derived from zinc, beryllium,alkaline earth metals such as magnesium, calcium, strontium, whereinmagnesium is preferred. Further applicable cations with higher chargeare cations from aluminium, iron, chromium, manganese, titanium,zirconium and other transition metals as well as double salts of suchcations or mixtures of the named salts. The use of aluminium salts andalums and various hydrates thereof such as e.g. AlCl₃×6 H₂O,NaAl(SO₄)₂×12H₂O, KAl(SO₄)₂×12H₂O or Al₂(SO₄)₃×14-18H₂O is preferred.

The use of Al₂(SO₄)₃ and its hydrates as cross-linkers of thecross-linker class IV is particularly preferred.

Preferred polymer particles are polymer particles, which arecross-linked by cross-linkers of the following cross-linker classes orby cross-linkers of the following combinations of cross-linker classes:I, II, III, IV, I II, I III, I IV, I II III, I II IV, I III IV, IV IIIIV, II IV or III IV. The above combinations of cross-linker classesproduce respectively a preferred embodiment of cross-linkers of apolymer particle.

Further preferred embodiments of the polymer particles are polymerparticles, which are cross-linked by any of the above namedcross-linkers of cross-linker class I. Among these, water solublecross-linkers are preferred. In this context,N,N′-methylenebisacrylamide, polyethyleneglycoldi(meth)acrylate,triallylmethylammonium chloride, tetraallylammonium chloride as well asallylnonaethyleneglycolacrylate made with 9 mol ethylene oxide per molacrylic acid are particularly preferred.

The water-absorbent polymers can be produced from the above-namedmonomers and cross-linkers by various polymerization means. For example,in this context can be named bulk polymerization which occurs preferablyin kneading reactors such as extruders, belt polymerization, solutionpolymerization, spray polymerization, inverse emulsion polymerizationand inverse suspension polymerization. Solution polymerization ispreferably carried out in water as solvent. The solution polymerizationcan occur continuously or discontinuously. From the prior art a broadspectrum of variation possibilities can be gathered with respect toreaction proportions such as temperature, type and quantity of theinitiators as well as of the reaction solution. Typical processes aredescribed in the following patent specifications: U.S. Pat. No.4,286,082, DE 27 06 135, U.S. Pat. No. 4,076,663, DE 35 03 458, DE 40 20780, DE 42 44 548, DE 43 23 001, DE 43 33 056, DE 44 18 818.

The polymerization is initiated by an initiator as is generallycustomary. All initiators forming radicals under the polymerizationconditions can be used as initiators for the initiation of thepolymerization, which initiators are customarily used in productionsuperabsorbers. An initiation of the polymerization by action ofelectron beams on the polymerizable aqueous solution is also possible.The polymerization can be initiated in the absence of initiators of theabove-mentioned type by action of energetic beams in the presence ofphoto-initiators. Polymerization initiators can be used dissolved ordispersed in a solution of monomer according to the invention. Allcompounds known to one experienced in the art to decompose into radicalscan be used as initiators. Hereunder fall in particular peroxides,hydroperoxides, hydrogen peroxide, persulfates, azo compounds as well asthe so-called redox catalysts. Preferred is the use of water solublecatalysts. In some cases it is advantageous to use mixtures of differentpolymerization initiators. Among these mixtures those initiatorscomprising hydrogen peroxide and sodium or potassium peroxodisulfate,which can be used in any conceivable quantity ratio, are preferred.Suitable organic peroxides are preferably acetylacetone peroxide,methylethylketone peroxide, t-butylhydroperoxide, cumolhydroperoxide,t-amylperpivalate, t-butylperpivalate, t-butylpemeohexonate,t-butylisobutyrate, t-butylper-2-ethylhexenoate, t-butylperisononanoate,t-butylpermaleate, t-butylperbenzoate, t-butyl-3,5,5-trimethylhexanoateand amylperneodecanoate. Additionally preferred as polymerizationinitiators are: azo compounds, such as2,2′-azobis(2-amidinopropane)-dihydrochloride,azo-bisamidinopropane-dihydrochloride,2,2′-azobis(N,N-dimethylene)isobutyramidine-dihydrochloride,2-(carbamoylazo)isobutyronitrile and 4,4′-azobis(4-cyanovaleric acid).The compounds mentioned are used in normal quantities, preferably withina range from about 0.001 to about 5, preferably from about 0.1 to about2 mol %, respectively based on the quantity of the monomers to bepolymerized.

The redox catalysts have as oxidic components at least one of theabove-indicated per-compounds and as reducing components preferablyascorbic acid, glucose, sorbose, mannose, ammonium or alkali metalhydrogensulfite, -sulfate, -thiosulfate, -hyposulfite or -sulfide, metalsalts, such as iron(II) ions or silver ions or sodiumhydroxymethylsulfoxylate. Preferably used as reducing components of theredox catalysts are ascorbic acid or sodium pyrosulfite. Based on thequantity of monomers to be used in the polymerization, about 1×10⁻⁵ toabout 1 mol % of the reducing component of the redox catalyst and about1×10⁵ to about 5 mol % of the oxidizing component of the redox catalystare used. In place of the oxidizing components of the redox catalyst, orin addition thereto, one or more preferably water soluble azo compoundscan be used.

If the polymerization is initiated by action of energetic beams,so-called photo-initiators are generally used. These can comprise forexample so-called α-splitters, H-abstracting systems or also azides.Examples of such initiators are benzophenone derivatives such asMichler's ketone, phenanthrene derivatives, fluorine derivatives,anthraquinone derivatives, thioxanthone derivatives, cumarinderivatives, benzoinether and derivatives thereof, azo compounds such asthe above-mentioned radical formers, substituted hexaarylbisimidazolesor acylphosphine oxides. Examples of azides are:2-(N,N-dimethylamino)ethyl-4-azidocinnamate,2-(N,N-dimethylamino)ethyl-4-azidonaphthylketone,2-(N,N-dimethylamino)ethyl-4-azidobenzoate,5-azido-1-naphthyl-2′-(N,N-dimethylamino)ethylsulfone,N-(4-sulfonylazidophenyl)maleinimide, N-acetyl-4-sulfonylazidoaniline,4-sulfonylazidoaniline, 4-azidoaniline, 4-azidophenacyl bromide,p-azidobenzoic acid, 2,6-bis(p-azidobenzylidene)cyclohexanone and2,6-bis(p-azidobenzylidene)-4-methylcyclohexanone. The photo-initiators,when used, are generally employed in quantities from about 0.01 to about5 wt. % based on the monomers to be polymerized.

A redox system used preferentially according to the invention compriseshydrogen peroxide, sodium peroxodisulfate and ascorbic acid. Generallyazo compounds are preferred as initiators according to the invention,wherein azo-bis(amidinopropane) dihydrochloride is particularlypreferred. As a rule the polymerization is initiated with the initiatorsin a temperature range of about 30 to about 90° C.

After producing the water-absorbent polymers these polymers are driedand broken up to form the above-described polymer particles.

As water soluble polymers (α4), water soluble polymerizates such asthose comprising partly or fully saponified polyvinyl alcohol,polyvinylpyrrolidone, starches or starch derivatives, polyglycols orpolyacrylic acids can preferably be polymerized into the polymerparticles. The molecular weight of these polymers is not critical, aslong as they are water soluble. Preferred water soluble polymers arestarches or starch derivatives or polyvinyl alcohol. The water solublepolymers, preferably synthetic polymers like polyvinyl alcohol, can alsoserve as graft basis for the monomers to be polymerized.

Additives (α5) to the polymer particles used in the process according tothe invention can comprise preferably suspension agents, odour binders,surface-active agents, or antioxidants. These additives (α5) arepreferably added before the polymerization of the monomer solution ormixed with the polymer particles after producing said particles, whereinfor the mixing, mixing aggregates known to one skilled in the art can beused, for example the Patterson-Kelley mixer, DRAIS turbulence mixer,Lödige mixer, Ruberg mixer, screw mixer, pan mixer and fluidized bedmixer as well as continually functioning vertical mixers, in which thepolymer particles and the additives (α5) are mixed with a fast frequencyby means of rotating knives (Schugi mixer).

In another embodiment of the process according to the invention theouter portion of the absorbent polymer particles is brought into contactwith a compound comprising Al³⁺ ions. Therein it is preferred that thecompound comprising Al³⁺ ions in a quantity within a range from about0.01 to about 30 wt. %, particularly preferred in a quantity within arange from about 0.1 to about 20 wt. % and above all preferred in aquantity within a range from about 0.3 to about 5 wt. %, respectivelybased on the weight of the absorbent polymer particles, is brought intocontact with the polymer particles.

Preferably the absorbent polymer particles are brought into contact withthe Al³⁺ ion-containing compound, by bringing this compound, in the formof a fluid comprising the Al³⁺ ion-containing compound as well as asolvent such as methanol or ethanol or mixtures of at least twotherefrom, into contact with the polymer particles. The Al³⁺ion-containing compound is thereby present in the fluid, withoutconsideration of water of crystallization, preferably in a quantitywithin a range from about 0.1 to about 50 wt. %, preferably in aquantity within a range from about 1 to about 30 wt. %, respectivelybased on the total weight of the fluid. It is further preferred that thefluid, in a quantity within a range from about 0.01 to about 15 wt. %,preferably in a quantity within a range from about 0.05 to about 6 wt. %respectively based on the weight of the absorbent polymer particles, isbrought into contact with the absorbent polymer particles.

Preferred compounds comprising Al³⁺ ions are AlCl₃×6 H₂O, NaAl(SO₄)₂×12H₂O, KAl(SO₄)₂×12 H₂O or Al₂(SO₄)₃×14-18H₂O.

In a further embodiment of the process according to the invention theliquid, with which in the first mixing event the polymer particles aremixed and which in the second mixing event is homogenized within thepolymer particles, comprises an Al³⁺ ion-containing compound. PreferredAl³⁺ ion-containing compounds are AlCl₃×6 H₂O, NaAl(SO₄)₂×12 H₂O,KAl(SO₄)₂×12 H₂O or Al₂(SO₄)₃×14-18 H₂O.

It is furthermore preferred according to the invention that the polymerparticles used in the process according to the invention have an innerportion, an outer portion surrounding the inner portion as well as asurface portion surrounding the outer portion, wherein the outer portionhas a higher degree of cross-linking than the inner portion, so thatpreferably a nucleus-shell structure forms. The increased cross-linkingin the surface portion of the polymer particle used is thereinpreferably accomplished by secondary cross-linking of reactive groupsclose to the surface. This secondary cross-linking can occur thermally,photochemically or chemically. As secondary cross-linkers for thechemical secondary cross-linking are therein preferred the compounds(α3) which were mentioned as cross-linkers of the cross-linker classesII and IV. Ethylene carbonate is particularly preferred as secondarycross-linker.

The secondary cross-linkers are preferably used in the cross-linking ina quantity within a range from about 0.01 to about 30 wt. %,particularly preferably in a quantity within a range from about 0.1 toabout 20 wt. % and above all preferably in a quantity within a rangefrom about 0.3 to about 5 wt. % respectively based on the weight of thepolymer particles used in the process according to the invention.

The secondary cross-linking preferably occurs thus, that a fluid F₁comprising a solvent, preferably water, water-miscible organic solventssuch as methanol or ethanol or mixtures of at least two thereof, as wellas the secondary cross-linker, is brought into contact with the outerportion of the polymer particles at a temperature within a range fromabout 30 to about 300° C., particularly preferred within a range fromabout 100 to about 200° C. The bringing into contact therein occurspreferably by spraying the fluid onto the polymer particles and thenmixing the polymer particles which have been brought into contact withthe fluid F₁. Therein the secondary cross-linker in the fluid F₁ ispresent preferably in a quantity within a range from about 0.01 to about20 wt. %, particularly preferably in a quantity within a range fromabout 0.1 to about 10 wt. %, based on the total weight of the fluid F₁.It is further preferred that the fluid F₁ be brought into contact withthe polymer particles in a quantity within a range from about 0.01 toabout 50 wt. %, particularly preferred in a quantity within a range fromabout 0.1 to about 30 wt. %, respectively based on the weight of polymerparticles.

If the absorbent polymer particles are brought into contact with an Al³⁺ion-containing compound, it is further preferred that the polymerparticles in the case of a secondary cross-linking are brought intocontact with the Al³⁺ ion-containing compound before carrying out thesecondary cross-linking. Therein the absorbent polymer particles canfirst be brought into contact with a fluid comprising the Al³⁺ion-containing compound and then with a fluid comprising thecross-linker. It is also conceivable to bring the Al³⁺ ion-containingcompound and the cross-linker in a common fluid into contact with theabsorbent polymer particles and then to effect the secondarycross-linking by increasing the temperature. The decisive factor ismerely that the bringing into contact of the absorbent polymer particleswith the Al³⁺ ion-containing compound occurs before carrying out thesecondary cross-linking reaction.

It is furthermore preferred that the polymer particles used in theprocess according to the invention have at least one of the followingproperties:

-   -   (A) the maximum absorption of 0.9 wt. % NaCl solution according        to ERT 440.1-99 is within a range from at least about 10 to        about 1000, preferably from about 15 to about 500 and        particularly preferred from about 20 to about 300 g/g,    -   (B) the part extractable with 0.9 wt. % NaCl solution according        to ERT 470.1-99 amounts to less than about 30, preferably less        than about 20 and particularly preferred less than about 10 wt.        %, based on the polymer particles,    -   (C) The bulk density according to ERT 460.1-99 is within a range        from about 300 to about 1000, preferably about 310 to about 800        and particularly preferred about 320 to about 700 g/l,    -   (D) The pH value according to ERT 400.1-99 for 1 g of the        polymer particles in 1 l water is within a range from about 4 to        about 10, preferably about 5 to about 9 and particularly        preferably about 5.5 to about 7.5.    -   (E) The CRC value according to ERT 441.1-99 is within a range        from about 10 to about 100, preferably about 15 to about 80 and        particularly preferably about 20 to about 60 g/g.    -   (F) The AAP value according to ERT 442.1-99 under a pressure of        0.7 psi (50 g/cm²) is within a range from about 10 to about 60,        preferably about 15 to about 50 and particularly preferably        about 20 to about 40 g/g.    -   (G) The AAP value according to ERT 442.1-99 under a pressure of        0.3 psi (20 g/cm²) is within a range from about 10 to about 100,        preferably about 15 to about 60 and particularly preferably        about 20 to about 50 g/g.

The property combinations of two or more properties arising from theabove properties represent respectively preferred embodiments of theprocess according to the invention. Further particularly preferredembodiments are processes, in which the polymer particles used in theprocess according to the invention have the following properties orcombinations of properties depicted as alphabetic characters orcombinations of alphabetic characters: A, B, C, D, E, F, G, AB, AC, AD,AE, AF, AG, EF, EG, FG, ABC, ABD, ABE, ABF, ABG, ACD, ACE, ACF, ACG,ADE, ADF, ADG, AEF, AEG, CEF, CEG, EFG, ABCD, ABCE, ABCF, ABCG, ABDE,ABDF, ABDG, ACDE, ACDF, ACDG, ACEF, ACEG, ADEF, ADEG, CEFG, ACDEF,ACDEG, ABDEF, ABDEG, ABCEF, ABCEG, ACBDF, ACBDG, ABCDE, ABCDEF, ABCDEGor ABCDEFG, wherein the combination CEFG is particularly preferred andthe combination EF is even more preferred.

Referring now to the Figures, which illustrate preferred embodiments andparticularities which can occur individually or in combination, itshould be understood that the illustrations do not limit the inventionand should only exemplify the invention.

FIG. 1 shows that in the first mixing event a polymer particle (1) ismixed with liquid (2). The surface of the polymer particle (1) is thuswetted with liquid (2) and a liquid (2)—wetted polymer particle (3)formed. In the second mixing event the liquid (2) penetrates the wettedpolymer particle (3) into the inside of the polymer particle (1).Thereby the liquid (2) is homogeneously distributed within the polymerparticles. The polymer particle (1) swells somewhat. The homogenizationof the liquid (2) within the polymer particle (1) causes a fasterabsorption of water.

FIG. 2 shows as an example a collision between a polymer particle (1)and two stuck-together polymer particles (1′, 1″). Both of thestuck-together polymer particles (1′, 1″) are held together by variousfactors such as for example cohesion or adhesion of the liquid (2) andthe polymer particles (1′, 1″). An adhesion layer (5) between thesurfaces of both polymer particles (1′, 1″) causes a further bindingeffect between both polymer particles (1′, 1″). By means of the highspeed of the approaching polymer particle (1) the adhesion between bothstuck-together polymer particles (1′) and (1″) is overcome. Inparticular the adhesion layer is broken open. The result of thecollision is a separation of the one stuck-together polymer particle(1′) from the other stuck-together polymer particle (1″), so that afterthe collision (4) the polymer particles (1, 1′, 1″) continue to moveindividually. An even wetting of the surfaces is achieved by means ofthe collision.

The process according to the invention for producing an absorbentpolymer has a first mixing event, in which a plurality of absorbentpolymer particles (1) is mixed with a liquid (2) and a second mixingevent, in which the liquid (2) is homogenized within the polymerparticles (1), wherein the polymer particles (1) are mixed in the firstmixing event with a speed such that the kinetic energy of the individualpolymer particles (1) is on average larger than the adhesion energybetween the individual polymer particles (1), and the polymer particles(1) in the second mixing event are stirred at a lower speed than in thefirst mixing event. The different speeds effect a fluidization of thepolymer particles (1), which fluidization prevents a clumping of thepolymer particles (1) during the mixing event. The absorbent polymerthus prepared or a composite or a chemical product, comprising a polymerof this type, distinguishes itself by a particularly rapid swellingbehavior.

The invention comprises furthermore the absorbent polymers obtainable bythe process according to the invention.

The invention comprises also an absorbent polymer, which is preferablyobtainable by the process according to the invention, comprising waterin a quantity within a range from about 0.1 to about 20 wt. %,preferably within a range from about 0.5 to about 15 wt. % and above allpreferably in a range from about 1 to about 5 wt. %, respectivelydetermined according to the oven method according to ERT 430.1-99 andrespectively based on the total weight of the absorbent polymer, whichhas at least one of the following properties:

-   (A1) an AAP value according to ERT 442.1-99 under a pressure of 0.7    psi (50 g/cm²) within a range from about 10 to about 60, preferably    about 15 to about 50 and particularly preferably about 20 to about    40 g/g,-   (B1) an AAP value according to ERT 442.1-99 under a pressure of 0.3    psi (20 g/cm²) within a range from about 10 to about 100, preferably    about 15 to about 60 and particularly preferred about 20 to about 50    g/g,-   (C1) a CRC value according to ERT 441.1-99 within a range from about    10 to about 100 g/g, preferably within a range from about 15 to    about 50 g/g and particularly preferred within a range from about 17    to about 40 g/g,-   (D1) a drop determined according to the herein-described method of    the AAP value determined according to ERT 442.1-99 under a pressure    of 0.7 psi after a degradation through mechanical stress of less    than about 20%, particularly preferred of less than about 15 wt. %    and above all preferred of less than about 10%,-   (E1) in a composite of 50 wt. % of the absorbent polymer, 47.5 wt. %    of cellulose fibers and 2.5 wt. % of a two-component fibre of    polypropylene and polyethylene an absorption time after a first    wetting, determined according to the herein described test method,    of less than about 53 seconds, preferably less than about 50 seconds    and particularly preferred less than about 46 seconds,-   (F1) in a composite of 50 wt. % of the absorbent polymer, 47.5 wt. %    of cellulose fibers and 2.5 wt. % of a two-component fiber of    polypropylene and polyethylene an absorption time after a second    wetting, determined according to the herein described test method,    of less than about 253 seconds, preferably less than about 225    seconds and particularly preferred less than about 200 seconds,-   (G1) in a composite of 50 wt. % of the absorbent polymer, 47.5 wt. %    of cellulose fibers and 2.5 wt. % of a two-component fiber of    polypropylene and polyethylene an absorption time after a third    wetting, determined according to the herein described test method,    of less than about 475 seconds, preferably less than about 450    seconds and particularly preferred less than about 400 seconds,-   (H1) in a composite of 50 wt. % of the absorbent polymer, 47.5 wt. %    of cellulose fibers and 2.5 wt. % of a two-component fiber of    polypropylene and polyethylene a rewet value, determined according    to the herein described test method, of less than about 12.55 g/g,    preferably less than about 12 g/g and particularly preferably less    than about 11 g/g,    wherein the water is homogeneously distributed in the absorbent    polymer.

Preferred absorbent polymers are those polymers which are characterizedby the following properties or property combinations: A1, B1, C1, D1,E1, F1, G1, H1, A1B1, A1C1, A1D1, B1C1, B1D1, C1D1, A1B1C1, A1B1D1,A1C1D1, B1C1D1, A1B1C1D1, E1F1, E1F1G1, F1G1, E1F1G1H1, F1G1H1, G1H1,D1E1F1G1H1, D1E1F1G1, D1E1F1, D1E1, D1H1, wherein D1 is particularlypreferred.

It is furthermore preferred, that the absorbent polymers according tothe invention have the same properties as the polymers obtainable by theprocess according to the invention. It is furthermore preferred, thataccording to an embodiment according to the invention of the processaccording to the invention as well as the absorbent polymer according tothe invention the values of characteristics according to the inventiongiven only with a lower limit have an upper limit, which is about 20times, preferably about 10 times and particularly preferably about 5times the most preferred value of the lower limit.

The invention also comprises a composite comprising the absorbentpolymers according to the invention as well as a substrate. It ispreferred that the absorbent polymers according to the invention and thesubstrate are securely bound together. As substrate are preferred filmsmade out of polymers, such as for example out of polyethylene,polypropylene or polyamide, metals, fleece, fluff, tissues, fabric,natural or synthetic fibers, or other foams.

Preferred composites according to the invention are sealant materials,cables, absorbent cores as well as diapers and hygiene articlescomprising them.

Sealant materials are preferably water-absorbent films, wherein theabsorbent polymer is worked into a polymer matrix or fiber matrix assubstrate. This is carried out preferably by mixing the absorbentpolymer with a polymer or fiber matrix-forming polymer (Pm) and finallybinding them, optionally by thermal treatment. In the case where theabsorbent structure is used as fibers, threads/yarns can be obtainedtherefrom which can be spun with additional fibers comprising anothermaterial as substrate and then for example bound together by knitting orweaving or be directly bound together, i.e. without being spun withadditional fibers. Typical processes herefor are described in H. Savanoet al., International Wire & Cable Symposium Proceedings 40, 333 to 338(1991); M. Fukuma et al., International Wire & Cable SymposiumProceedings, 36, 350 to 355 (1987) and in U.S. Pat. No. 4,703,132.

In the embodiment in which the composite is a cable, the absorbentpolymer as particles can be directly used, preferably beneath theinsulation of the cable. In another embodiment of the cable theabsorbent polymer (Pa) can be used in the form of swellabletension-resistant yarns. According to another embodiment of the cablethe absorbent polymer can be used as swellable film. Furthermore inanother embodiment of the cable the absorbent polymer can be used asmoisture-absorbent cores in the middle of cables. The substrate in thecase of the cable forms all components of the cable which contain noabsorbent polymer. Hereunder are included conduits, such as electricallines or light conduits, optical or electrical insulation materials aswell as components of the cable which ensure the mechanicalapplicability of the cable, such as networks, fibers or fabrics madefrom tension-resistant materials such as synthetic materials andinsulators made from rubber or other materials which prevent thedestruction of the exterior of the cable.

If the composite is an absorbent core, the absorbent polymer is workedinto a substrate. This substrate can preferably be in the form offibrous materials. Fibrous materials which can be used in the presentinvention comprise natural fibers (modified or unmodified) as well assynthetic fibers. Examples of suitable unmodified and modified naturalfibers comprise cotton, Esparto grass, sugarcane, kemp, flax, silk,wool, cellulose, chemically modified pulp, jute, rayon, ethylcelluloseand cellulose acetate. Suitable synthetic fibers can be produced frompolyvinylchloride, polyvinylfluoride, polytetrafluoroethylene,polyvinylidenechloride, polyacrylates such as Orion®, polyvinylacetate,polyethylvinylacetate, insoluble or soluble polyvinylalcohol,polyolefins such as polyethylene (for example PULPEX®) andpolypropylenes, polyamides such as nylon, polyesters such as DACRON® orKodel®, polyurethanes, polystyrenes and the like. The fibers used cancomprise only natural fibers, only synthetic fibers or any compatiblecombination of natural and synthetic fibers.

The fibers used in the present invention can be hydrophilic orhydrophobic, or they can comprise a combination of hydrophilic andhydrophobic fibers. The term “hydrophilic” as used here describes fibersor surfaces of fibers which can be wetted by aqueous liquids (forexample aqueous body liquids) which are deposited on these fibers.Hydrophilicity and wettability are typically defined in terms of thecontact angle and the surface tension of the concerned liquids andsolids. This is discussed in detail in a publication of the AmericanChemical Society with the title “Contact Angle, Wettability andAdhesion”, published by Robert F. Gould (copyright 1964). A fiber or thesurface of a fiber is wetted by a liquid (i.e. it is hydrophilic) ifeither the contact angle between the liquid and the fiber or the surfacethereof amounts to less than about 90°, or if the liquid tends todistribute itself spontaneously over the surface, wherein bothconditions are normally simultaneous. On the other hand a fiber or thesurface of a fiber is considered as hydrophobic, if the contact angle islarger than about 90° and the liquid does not spread spontaneously onthe surface of the fiber.

Preferred fibers according to the invention are hydrophilic fibers.Suitable hydrophilic fibers comprise cellulose fibers, modifiedcellulose fibers, rayon, polyester fibers, such aspolyethyleneterephthalate (for example DACRON®), hydrophilic nylon(HYDROFIL®) and the like. Suitable hydrophilic fibers can also beobtained by hydrophilising hydrophobic fibers, such as surface-activeagent-treated or silica-treated thermoplastic fibers, which are derivedfor example from polyolefins such as polyethylene or polypropylene or onpolyacrylates, polyamides, polystrenes, polyurethanes and the like. Forreasons of availability and of cost cellulose fibers, in particular pulpfibers, are preferred for use in the present invention. Furtherpreferred hydrophilic fibers for use in the present invention arechemically stiffened cellulose fibers. The term “chemically stiffenedcellulose fibers” describes therein cellulose fibers which are stiffenedby means of a chemical medium in order to increase the stiffness of thefibers under dry as well as under aqueous conditions. Such media cancomprise a chemical stiffening agent, which for example covers and/orimpregnates the fibers. Such a medium can also comprise the stiffeningof the fibers by changing the chemical structure, for example bycross-linking of polymer chains. Polymer-stiffening agents which cancover or impregnate the cellulose fibers comprise: cationic starcheswhich have nitrogen-containing groups (for example amino groups), whichare obtainable from the National Starch and Chemical Corp., Bridgewater,N.J., USA, latexes, moisture-resistant resins such as polyamideepichlorohydrin resin (for example Kymene® 557H, Hercules, Inc.,Wilmington, Del., USA), polyacrylamide resins, as described for examplein U.S. Pat. No. 3,556,932, commercially available polyacrylamides suchas Parez® 631 NZ of the American Cyanamid Co., Stanfort, Conn., USA,ureaformaldehydes as well as melamineformaldehyde resins. Fibers whichwere stiffened by cross-linking connections in individual forms (i.e.the individually stiffened fibers as well as the process for theirproduction) are for example described in U.S. Pat. No. 3,224,926, U.S.Pat. No. 3,440,135, U.S. Pat. No. 3,932,209 as well as in U.S. Pat. No.4,035,147. Preferred cross-linking agents are glutaraldehyde, glyoxal,formaldehyde, glyoxalic acid, oxydisuccinic acid and citric acid. Thestiffened cellulose fibers obtained by cross-linking or coating,impregnation or cross-linking can be twisted or curled, the fibers arepreferably twisted and additionally curled.

Besides the above-mentioned fibrous materials the core can also includethermoplastic materials. On melting, at least a part of thisthermoplastic material, typically because of the capillary gradient,penetrates between the fibers to the intersections of the fibers. Theseintersections become binding positions for the thermoplastic material.If the element is cooled, the thermoplastic material solidifies at theseintersections, to form binding positions which hold together the matrixor the tissue of fibers in each of the respective layers. Thethermoplastic materials can be in various forms, such as particles,fibers or combinations of particles and fibers. These materials cancomprise a plurality of thermoplastic polymers, selected frompolyolefins, such as polyethylene (for example PULPEX®) andpolypropylene, polyesters, co-polyesters, polyvinylacetates,polyethylvinylacetates, polyvinylchlorides, polyvinylidenechlorides,polyacrylates, polyamides, co-polyamides, polystyrenes, polyurethanesand copolymers of the above materials, such asvinylchloride/vinylacetate and the like. For cores, predominantlymaterials made from cellulose, preferably fibrous, can be used assubstrate.

In a further embodiment of the core this core comprises besides thesubstrate and the absorbent polymer further substances in the form ofpowders, such as for example odour binding substances such ascyclodextrins, zeolites, inorganic or organic salts or similarmaterials.

In one embodiment of the absorbent core the absorbent polymer is workedin a quantity within the range from about 10 to about 90, preferablyfrom about 20 to about 80 and particularly preferably from about 40 toabout 70 wt. %, based on the core. In one embodiment of the core theabsorbent polymer is worked into the core as particles. Thereby thepolymer particles can be homogeneously distributed in the fibrousmaterial, they can be positioned in layered fashion between the fibrousmaterial or the concentration of the absorbent polymer particles canhave a gradient within the fibrous material. In another embodiment ofthe core the absorbent polymer is worked into the core as fibers.

Optionally several different absorbent polymer particles, which differfor example in the rate of absorption, in the permeability, in theretention capacity, in the absorption against pressure, the graindistribution or also in the chemical composition, can be employedsimultaneously. These various polymer particles can be put already mixedtogether into the absorbent pad or positioned in the core with localdifferentiations. Such a differential positioning can occur in thedirection of the thickness of the core or of the length or breadth ofthe core.

The core can be combined by conventional processes known to one skilledin the art, such as to one skilled in the art generally amongdrum-forming, by means of shaping wheels, -pockets and product forms andappropriately adapted dosing arrangements. Besides this there aremodern, established process such as the so-called airlaid processes(e.g. EP 850 615, U.S. Pat. No. 4,640,810) with all forms of the dosing,depositing of the fibers and consolidation such as hydrogen bonding(e.g. DE 197 50 890), thermo-bonding, latex bonding, (e.g. EP 850 615)and hybrid bonding, the so-called wetlaid processes (e.g. WO 99/49905),carding processes, meltblown processes, spunblown processes as well assimilar processes for producing superabsorbent non-wovens (in the senseof the definition of the EDANA, Brussels) also in combinations of theseprocesses with and among usual methods for producing the cores. Furtherprocesses which could be used are the production of laminates in thebroadest sense as well as of extruded and co-extruded, wet- and dry- aswell as additionally reinforced structures.

In a further embodiment of the absorbent core this core comprisesbesides the substrate and the absorbent polymer worked into thesubstrate, which serve as storage layer for the body liquids, anabsorbent layer which preferably serves to absorb and distribute quicklythe liquid in the core. Thus the absorption layer can be arrangeddirectly over the storage layer, it being however also possible that theabsorption layer is separated from the storage layer by a preferablyliquid-stable interface. This interface serves then in the firstinstance as support substrate for the absorption layer and the storagelayer. Preferred materials for this interface are polyester spun fleecesor fleeces made from polypropylene, polyethylene or nylon.

In one embodiment of the core according to the invention the absorbentlayer is free from absorbent polymer. The absorbent layer can have anysuitable size and must not exceed the total length or breadth of thestorage layer. The absorbent layer can for example be in the form of astrip or a patch. The total absorbent layer is preferably hydrophilicbut can also have hydrophobic components. The absorbent layer cancomprise a woven material, a fleece material, or another suitable typeof material. The absorbent layer is preferably based on hydrophobicpolyethylene-terephthalate fibers (PET fibers), chemically stiffenedcellulose fibers or on mixtures of these fibers. Further suitablematerials are polypropylene, polyethylene, nylon or biological fibers.If the absorbent layer comprises a fleece material, said layer can beproduced by a multiplicity of different processes. These comprisewet-laying, application in an air stream, application in a melt, formingas spun fleece, carding, (this comprises thermal joining, joining withsolvents or joining with the melt-spin process). The last mentionedprocesses (forming as spun fleece and carding) are preferred when it isdesirable to align the fibers in the absorbent layer, since it is easierin such processes to align the fibers in a single direction. Aparticularly preferred material for the absorbent layer is a PET-spunfleece.

In the embodiment in which the composite is a diaper, the components ofthe diaper which are different to the absorbent polymer comprise thesubstrate of the composite. In a preferred embodiment the diapercomprises an above-described core. In this case the components of thediaper which are different to the core comprise the substrate of thecomposite. In general a composite used as a diaper comprises awater-impermeable lower layer, a water-permeable, preferablyhydrophobic, upper layer and a layer comprising the absorbent polymer,which is arranged between the lower layer and the upper layer. Thisabsorbent polymer-comprising layer is preferably a heretofore-describedcore. The lower layer can comprise all materials known to one skilled inthe art, wherein polyethylene or polypropylene are preferred. The upperlayer can likewise comprise all suitable material known to one skilledin the art, wherein polyesters, polyolefins, viscose and the like arepreferred, which give a sufficiently porous layer to ensure asatisfactory liquid permeability of the upper layer. In this contextreference is made to the disclosure in U.S. Pat. No. 5,061,295, US Re.26,151, U.S. Pat. No. 3,592,194, U.S. Pat. No. 3,489,148 as well as U.S.Pat. No. 3,860,003.

The invention further comprises a process for producing a composite,wherein an absorbent polymer according to the invention and a substrateand optionally a suitable additive are brought into contact with eachother. The bringing into contact occurs preferably by wetlaid andairlaid processes, compression, extrusion and mixing.

In addition the invention comprises a composite which is obtainable bythe above processes.

The invention is further related to chemical products, in particularfoams, formed bodies, fibers, sheets, films, cables, sealant materials,liquid-absorbing hygiene articles, carriers for plant or mushroom growthregulating media or plant protection agents, additives for buildingmaterials, packing materials or soil additives, which comprise theabsorbent polymer according to the invention or the above-describedsubstrate.

The invention is additionally related to the use of absorbent polymeraccording to the invention or of the above described substrate inchemical products, in particular in foams, formed bodies, fibers,sheets, films, cables, sealant materials, liquid-absorbing hygienearticles, carriers for plant or mushroom growth-regulating media orplant protection agents, additives for building materials, packingmaterials or soil additives.

In the use as carriers for plant or mushroom growth-regulating media orplant protection agents, it is preferred that the plant or mushroomgrowth-regulating media or plant protection agents can be released overa time period controlled by the carrier.

The invention is now more closely illustrated by means of test methodsand non-limiting examples.

Test Methods Determination of the Drop in the AAP Value

In order to determine the drop in the AAP value determined according toERT 442.1-99 under a pressure of 0.7 psi as a result of a mechanicaltreatment of the polymer particles in a ball mill the AAP value (AAP₁)of the polymer particles ERT 442.1-99 under a pressure of 0.7 psi wasfirst determined. Then these polymer particles were exposed to damagevia mechanical stress. To this end 10.0 g±0.1 g superabsorber wereweighed into a porcelain container with 24 porcelain cylinders andsealed. Two porcelain containers were then placed on the rollers of aball mill (Jar mill, US-Stoneware), which was set up by means of aphoto-contact tachometer such that 95 rpm were reached, and the ballmill is started. The rotation time was ended by means of a stop-watchafter exactly 6 minutes. Thereafter the AAP value (AAP₂) of thethus-treated polymer particles under a pressure of 0.7 psi wasdetermined again. The drop in the AAP value is defined as

drop in AAP value [%]=100%−((AAP₂/AAP₁)×100)

wherein respectively the average value for the AAP₂ or AAP₁ value from20 individual measurements for the determination of the decrease in theAAP value was used.

Determination of the Absorption Time (Acquisition Time)

In order to determine the absorption time a body-shaped test device wasused, the cross-section of which is shown in FIG. 3. This consists of abody-shaped semi-cylindrical element (6) with a length of 20.5 cm with around opening (7) and an inlet pipe (8). All surfaces of the employedparts of the test device are smooth. As can be seen from FIG. 4, whichdepicts a detailed view of the opening (7), this opening (7) has a sieve(9) with a mesh size of 3 to 4 mm. The element (6) was installed in alikewise essentially semicircular bowl (10). Between the element (6) andthe bowl (10) in the region of the opening (7) of the inlet pipe (8) thetest body (11), also named composite, was inserted. The exact dimensionsof the element (6), the opening (7) as well as the inlet pipe (8), canbe seen in FIG. 5. Furthermore in the measurement of the absorption timea balance with a measurement precision of 0.01 as well as weights forloading the test apparatus of one Kilo±0.5 g or pneumatic plungers, atimer, a round filter paper 90 mm for quantitative analysis from thefirm Schleicher & Schuell, Type 589/1, Schwarzbrand as well as a 0.9 wt.% sodium chloride-aqua dest. test solution, which was coloured with 5ml/l acid fuchsine stock solution. The composite to be tested was cut toa size 12 cm by 30 cm and laid in the centre of the body-shaped testapparatus between the element (6) and the bowl (10). The element (6) wasloaded with 9 kg. Three times 80 g of the test solution was fed throughthe inlet pipe (8) via the opening (7) to the composite to be tested inintervals of 20 minutes, wherein the 20 minute interval begins with theaddition of the test solution and the test solution was put all at onceinto the inlet pipe. The measurement of the absorption time started withthe completion of the addition of the test solution and ended with thefull seepage away of the test solution in the inlet pipe. The abovemeasurement process was repeated 3 times and the respective averageobtained from the thus-obtained values.

Determination of the Rewet Value

In order to determine the rewet value an experimental set-up accordingto FIGS. 6 and 7 was used. The composite (11) to be measured was laid inthe form of a circular test item with a diameter of 9 cm, of which theweight was determined, in the middle of a petri dish (12). By means of aburette (13) 80 g of the previously described test solution were addedto the centre of the test item with a flow rate of 1.8-2.0 ml/s. Afterstopping the addition of the test solution a likewise circular filterpaper stack (14), weighing 40.0 g and with a diameter of 9 cm, ofSchwarzbrand filter paper for quantitative analysis of the firmSchleicher & Schuell Type 58971, was weighed (F_(T)) and carefully laidon the top side of the test item. The filter paper stack (14) was loadedby means of weight (15) with 1,270 g to ca. 20 g/mm² for 10 minutes.After stopping the loading the filter paper stack (14) was carefullyremoved from the test item and weighed again (F_(F)). The rewet value isdetermined by F_(T)-F_(F) [g]. The measurement is repeated 5 times andthe average value obtained.

EXAMPLES To Produce a Secondary Cross-Linked Absorbent Polymer

1.50 g Polyethyleneglycol-300-diacrylate and 1.50 gpolyethyleneglycol-750-monoallyletheracrylate as cross-linkers aredissolved in 964.515 g of an aqueous solution of sodium acrylate with adegree of neutralization of 70 mol % (monomer concentration: 37.7%). Themonomer solution in a plastic polymerization vessel was flushed withnitrogen for 30 minutes in order to remove dissolved oxygen. At atemperature of 4° C. the polymerization was started by successiveaddition of 0.3 g sodium peroxidisulfate in 10 g distilled water, 0.1 g2,2′-azobis-2-amidinopropane-dihydrochloride in 10 g dist. water, 0.07 g35% hydrogen peroxide solution in 10 g dist. water and 0.015 g ascorbicacid in 2 g dist. water. After the end temperature of ca. 100° C. wasreached, the gel was broken up with a mincer and dried for 2 h at 150°C. in a convection oven. The dried product was coarsely ground, finelyground and the particles with sizes from 150-850 μm sieved out forfurther transformation (powder A).

Comparison Example 1

50 g Powder A was mixed with vigorous stirring with a solution of 0.15 galuminium sulfate 18-hydrate and 0.15 g water and then with a solutionof 0.3 g ethylene carbonate and 0.3 g water and finally kept in aconvection oven at 170° C. for 90 minutes (powder B). Powder B ischaracterized by the properties listed in Table 1.

TABLE 1 Property of powder B Unit Measurement result CRC g/g 28.7 AAP(0.7 psi) g/g 21.6 Decrease in AAP (0.7 psi) % 24

Example 1

200 g of the secondary cross-linked absorbent polymer (powder B) was putinto an rotation jar of a rapidly rotating MTI mixer (MTI mixingtechnology, Industrieanlagen GmbH, 322758 Detmold: type LM1.5/5; year ofconstruction 1995) which rotates (volume ca. 11) with a speed of themixing aggregate of 1000 rpm. 4 ml Deionized water was then measured inby syringe. After ca. 5 to 15 seconds the liquid and the absorbentpolymer are homogeneously mixed. After a waiting time of 15 minutes(maturation) a second agitation event was carried out with aKrups-Drei-Mixer 3000 at level 1 (slowest revolution speed) over 5minutes, hereby those particles still adhered to each other wereseparated from each other and a free-flowing, clump-free materialobtained (powder C). Powder C is characterized by the properties listedin Table 2.

TABLE 2 Property of powder C Unit Measurement result CRC g/g 28.8 AAP(0.7 psi) g/g 21.2 Decrease in AAP (0.7 psi) % 4.8

Preparation of Composites for Test Items

The composites were formed by means of a mixture of 50 wt. % of anabsorbent polymer (powder B or C), based on the composite, and 47.5 wt.% cellulose fibers Stora Fluff EF semitreated from Stora-Enzo AB Sweden,as well as 2.5 wt. % of a two-component fiber of respectively 50 wt. %polypropylene (PP) or polyethylene (PE) with a PP core and a PE coatfrom the firm Fibervision A/S Denmark by an airlaid process with a M&Jmachine (width 40 cm, operating width 36 cm, operational set-up: bandspeed 2 m/min, fluff drawing-in at hammer mill 3.6 m/min, polymer dosage400 g/min, two-component fiber discharged in 10 g portions 0.7times/min), wherein the absorbent polymer was added homogeneously.Composites with a basis weight of 870 g/m² including tissue (1 layer, 18g/m²), with a density of 0.16 g/cm³ were used as test items for thefollowing tests. The results of the test carried out are summarized inTable 3.

TABLE 3 Absorption Absorption Absorption time after first time aftersecond time after third Rewet Sample wetting [s] wetting [s] wetting [s][g] Powder B 53 253 475 12.55 Powder C 45 197 399 10.48

1-13. (canceled)
 14. An absorbent polymer obtainable by a process forproducing an absorbent polymer comprising the steps of: a first mixingevent, wherein a plurality of absorbent polymer particles are mixed witha liquid in a mixer; and a second mixing event, wherein the liquid isdistributed within the polymer particles; wherein the polymer particlesin the first mixing event are mixed with a first mixing speed, and thepolymer particles in the second mixing event are stirred with a lowerspeed than in the first mixing event; and wherein the first mixing eventis a continuous mixing process wherein in the first mixing event thepolymer particles are back-mixed in such a way that a flow of the newpolymer particles entering in the mixer is overlaid by a flow of polymerparticles already present in the mixer and opposed to this flow.
 15. Anabsorbent polymer comprising water in a quantity within the range fromabout 0.1 to about 20 wt. % based on the total weight of the absorbentpolymer, which has at least one of the following properties: (A1) an AAPvalue under a pressure of 0.7 psi (50 g/cm²) within a range from about10 to about 60 g/g, (B1) an AAP value under a pressure of 0.3 psi (20g/cm²) within a range from about 10 to about 100 g/g, (C1) a CRC valuewithin a range from about 10 to about 100 g/g, (D1) a drop of the AAPvalue under a load of 0.7 psi of less than about 20% after adeterioration through mechanical stress, (E1) in a composite of 50 wt. %of the absorbent polymer, 47.5 wt. % cellulose fibers and 2.5 wt. % of atwo-component fiber of polypropylene and polyethylene an absorption timeafter a first wetting of less than about 53 seconds, (F1) in a compositeof 50 wt. % of the absorbent polymer, 47.5 wt. % cellulose fibers and2.5 wt. % of a two-component fiber of polypropylene and polyethylene anabsorption time after a second wetting of less than about 253 seconds,(G1) in a composite of 50 wt. % of the absorbent polymer, 47.5 wt. %cellulose fibers and 2.5 wt. % of a two-component fiber of polypropyleneand polyethylene an absorption time after a third wetting of less thanabout 475 seconds, (H1) in a composite of 50 wt. % of the absorbentpolymer, 47.5 wt. % cellulose fibers and 2.5 wt. % of a two-componentfiber of polypropylene and polyethylene a rewet value of less than about12.55 g/g, wherein the water is homogeneously distributed within theabsorbent polymer. 16-22. (canceled)
 23. The absorbent polymer accordingto claim 14, wherein the ratio of the opposed flow to the flow of newlyentering polymer particles averages from about 5 to about 50% by wt. 24.The absorbent polymer according to claim 14 wherein before the firstmixing event the absorbent polymer particles have been secondarycross-linked in the surface portion and have been brought into contactwith a composition comprising an Al³⁺ ion before the secondarycross-linking.
 25. The absorbent polymer according to claim 14 whereinthe average speed of the polymer particles in the first mixing eventamounts to from about 8 to about 80 m/sec.
 26. The absorbent polymeraccording to claim 14 wherein the average speed of the polymer particlesin the first mixing event amounts from about 15 to about 60 m/sec. 27.The absorbent polymer according to claim 14 wherein the average speed ofthe polymer particles in the first mixing event amounts from about 20 toabout 30 m/sec.
 28. The absorbent polymer according to claim 14 whereinthe average speed of the polymer particles in the second mixing eventamounts to less than about 3 m/sec.
 29. The absorbent polymer accordingto claim 14 wherein the average speed of the polymer particles in thesecond mixing event amounts to less than about 0.3 m/sec.
 30. Theabsorbent polymer according to claim 14 wherein the average speed of thepolymer particles in the second mixing event amounts to less than about0.03 m/sec.
 31. The absorbent polymer according to claim 14 wherein theFroude number in the first mixing event amounts from about 1 to about50.
 32. The absorbent polymer according to claim 14 wherein the Froudenumber in the first mixing event amounts from about 1.5 to about
 40. 33.The absorbent polymer according to claim 14 wherein the Froude number inthe first mixing event amounts from about 1.7 to about
 33. 34. Theabsorbent polymer according to claim 14 wherein the Froude number in thesecond mixing event amounts from about 0.001 to about
 1. 35. Theabsorbent polymer according to claim 14 wherein the Froude number in thesecond mixing event amounts from about 0.01 to about 0.2.
 36. Theabsorbent polymer according to claim 14 wherein the Froude number in thesecond mixing event amounts from about 0.08 to about 0.03.
 37. Theabsorbent polymer according to claim 14 wherein a back-mixing from about10% to about 30% occurs.
 38. The absorbent polymer according to claim 14wherein the average residence time of the first mixing event amountsfrom about 5 to about 200 sec.
 39. The absorbent polymer according toclaim 14 wherein the average residence time of the first mixing eventamounts from about 10 to about 100 sec.
 40. The absorbent polymeraccording to claim 14 wherein the average residence time of the firstmixing event amounts from about 20 to about 60 sec.
 41. The absorbentpolymer according to claim 14 wherein the static pressure build upduring the first mixing event amounts to less than about 0.1 bar. 42.The absorbent polymer according to claim 14 wherein water or aqueoussolution is added as liquid.
 43. The absorbent polymer according toclaim 42, wherein the liquid comprises additives.
 44. The absorbentpolymer according to claim 14, wherein the polymer particles are basedon: (α1) about 0.1 to about 99.999 wt % polymerized, ethylenicallyunsaturated, acidic group-containing monomers containing a protonated ora quaternary nitrogen, or mixtures thereof, (α2) 0 to about 70 wt % ofpolymerized, ethylenically unsaturated monomers which can beco-polymerized with (al), (α3) about 0.001 to about 10 wt % of one ormore cross-linkers, (α4) 0 to about 30 wt % of water soluble polymers,as well as (α5) 0 to about 20 wt % of one or more additives, wherein thesum of the component weights (α1) to (α5) amounts to 100 wt %.
 45. Theabsorbent polymer according to claim 14 wherein the polymer particleshave at least one of the following properties: (A) the maximumabsorption of 0.9 wt % NaCl solution is within a range from at leastabout 10 to about 1000 g/g SAP granulate, (B) the part extractable with0.9 wt % aqueous NaCl solution amounts to less than about 30 wt %, basedon the SAP granulate, (C) the bulk density is within a range from about300 to about 1000 g/l, (D) the pH value for 1 g of the SAP granulate in1 l water is within a range from about 4 to about 10, (E) the CRC valueis within a range from about 10 to about 100 g/g, (F) the AAP valueunder a pressure of 0.7 psi is within a range from about 10 to about 60g/g, or (G) the AAP value under a pressure of 0.3 psi is within a rangefrom about 10 to about 1000 g/g.